DROP MARKING CONTROL SYSTEM RESPONSIVE TO ACOUSTICAL PROPERTIES OF INK

Abstract: A control system (40) is disclosed for an ink jet printer. The control system employs acoustic transducers (12, 13, 15) to determine the velocity of sound in the ink and in a reference chamber (10) containing only fresh ink. By comparing the two measurements it can be determined when to add additional solvent to the system to maintain solids concentration substantially constant.

Description: DROP MARKING CONTROL SYSTEM RESPONSIVE TO ACOUSTICAL PROPERTIES OF INK This invention relates to the field of drop marking systems of the type in which a liquid ink is forced under pressure through a nozzle which converts the liquid into droplets which can then be controlled by various means while projected toward a substrate for marking purposes. Examples of such systems include the familiar ink jet marking systems used for high speed label printing, product identification and the like, although there are other drop marking systems known in the art. One particular type of system which advantageously employs the present invention is the continuous stream, ink jet printer. Such a system typically includes an ink reservoir and a remotely located nozzle connected to the reservoir by a conduit. Ink is forced under pressure from the reservoir to the nozzle which emits a continuous stream of ink drops. The ink, which is electrically conductive, is provided with a charge as the drops leave the nozzle. The drops then pass through a deflection field which causes selected drops to be deflected so that some of the drops are deposited onto a substrate while the remaining drops are returned to the reservoir by a suitable ink return means. In order to produce high quality marking, it is important that the ink is maintained at its formulated concentration of nonevaporative solids. Ink drop formation, drop electrical charge, drop velocity, spot placement accuracty, spot placement precision, spot adhesion, spot drying time, and spot optical properties are printing parameters that have some dependence on ink properties. These ink properties include compsition, electrical conductivity, density, acoustic velocity, surface tension, and viscosity. These ink properties have a dependence on solids concentration. So, if ink solids concentration deviates from specifications, the print quality may also deviate from acceptable standards. Print quality is highly dependent on drop velocity. In turn, drop velocity is dependent on ink viscosity. Ink viscosity is highly dependent on ink solids concentration. Thus, drop velocity and print quality are strongly dependent on ink solids concentration. The condition of constant ink drop velocity through the deflection field is necessary to provide consistent print quality and requires that the flow rate of liquid through the nozzle be substantially constant. Prior ink marking systems have attempted to accommodate this requirement by various means. One such system employs a specific gravity detector which signals when it is necessary to add solvent to the ink supply. This system is unsuitable for use in systems where the printer must accommodate many different types of inks, each with its own specific gravity parameters. Another commercial system which tries to deal with the problem of changing drop velocity was manufactured by the IBM Corporation. In this device the ink pressure is responsive to signals from a deflection detector. The deflection detector is located in the electric field through which the drops pass. The detector signals the pump to increase or decrease pressure, as necessary, to maintain drop velocity at an appropriate value. The system provides feedback control of drop velocity. The technique, however, is not entirely satisfactory because of the complexity and cost of the components and the need for a fragile deflection detector at the remote print head location. Another invention, disclosed in U.S. Patent No. 4,555,712, monitors the ink flow rate, monitors the velocity of the drops of ink in the charge field and, by use of an electronic controller, adjusts the ink parameters to maintain a desired flow rate which insures a substantially constant drop velocity. It is an object of the present invention to incorporate direct feedback control into an ink solids concentration control system which is simpler, reliable and low in cost. Another object of the invention is to provide a velocity control system for an ink jet printer which maintains substantially constant velocity of ink entering a deflection field thereby insuring accurate location of spots on the substrate to be marked. In a first aspect the present invention provides ink control apparatus for a drop marking system which includes a reservoir, a printhead receiving ink from the reservoir, means for returning unused ink to the reservoir, the apparatus comprising; a) a solvent supply including a valuve for controlling the addition of solvent to the ink in the reservoir; b) a reference chamber containing fresh ink; and characterised by further comprising; c) an acoustic transmitter associated with said reference chamber oriented to transmit acoustic pulses through the ink and to detect said acoustic pulses after travelling through the ink, the time delay between transmission of a pulse and its reception being a function of the solids concentration of the ink in the chamber; d) a second acoustic transmitter associated with said reservoir oriented to send acoustic pulses through the ink and a second acoustic receiver associated with said reservoir arranged to detect said acoustic pulses after travelling through the ink the time delay between transmission of a pulse and its reception being a function of the solids concentration of the ink in the reservoir; e) means for comparing the time delays of echo returns from the reference chamber and the reservoir and for controlling operation of the solvent supply valve as a function of the said difference; whereby solvent is added to the ink received in the reservoir as necessary to keep the solids concentration therein substantially the same as the ink in the reference chamber. Thus the invention provides an electronic control system employing acoustic transducers to measure the velocity of sound in ink to permit accurate control of the addition of solvent to the ink. The invention also provides a flow control means for an ink system which is located entirely separate from the print head nozzle and yet maintains a substantially constant flow rate through the nozzle. In a second aspect the present invention provides ink control apparatus for a drop marking system including a reservoir to supply ink to a print head that has at least one orifice for projecting a stream of droplets towards the surface to be marked, the individual droplets being electrically controlled and directed towards the surface to mark it or to a catcher to be captured and returned to the ink reservoir, and having a source of fresh ink and a source of solvent which is selectively connected to the ink reservoir to replenish it and characterised by ink comprising; a) first means measuring acoustic properties of the fresh ink to obtain a reference signal value relating to ink solids concentration. b) second measuring means measuring the acoustic properties of the captured ink to obtain a signal value representative of the ink solids concentration in the captured ink. c) means for comparing said reference signal value and said representative signal value to generate an error signal proportion to the solids concentration variation between the captured ink and the fresh ink, and d) means for controlling the addition of the solvent in response to said error signal whereby the solids concentration of the ink and the ink reservoirs maintained substantially constant. Other objects and advantage of the invention will be apparent from the remaining portion of the description. The present invention employs sound velocity measurement to determine ink concentration. A transducer is used to emit an acoustic pulse in a reference chamber fed directly by a fresh ink supply. This reference measurement is used as the control input to a feedback control system. A similar acoustical measurement is taken in either the return reservoir or the high pressure supply reservoir, or both, by additional transducers. These measurements are also fed to the control system, for example, a microprocessor, which determines the difference and directs appropriate adjustements in ink concentration by adding solvent or if fluid level is low, adds fresh ink. In addition to the previously mentioned advantages, this system e liminates the need for float based level sensors and evaporated loss measurements as are frequently used in the prior art to maintain fluid levels and viscosity. In the supply and return reservoirs the acoustic transducer will receive a second acoustic pulse reflected from the liquid surface indicative of fluid level and flow rate. While the resulting fluid level data is not required for concentration control, this data can be used to maintain optimal fluid levels and to provide flow rate measurements in the system. In a typical application, the ink flow rate and drop velocity are initially set using fresh ink by adjustment of the pressure in the ink flow line, to a condition which yields proper drop spacing. Thereafter the acoustic measurements from either the supply or return reservoirs coupled with signals from the reference chamber permit precise ink concentration control. Apparatus embodying the invention will now be described by way of example only with reference to the accompanying diagrammatic figures in which; Figure 1 is a schematic drawing of an ink jet printing system incorporating the elements of the present invention; Figure 2 is a block diagram of a closed-loop electronic control system suitable for practicing the invention; and Figure 3 is a waveform diagram showing the acoustic pulses and the relationship of the return pulses used to generate error signals. Referring to Figure 1, a generalized schematic of the invention, applied to a typical ink drop marking system, is shown. In a typical ink drop marking system a plurality of ink drops separated by a pre-determined spacing emanate from an ink jet nozzle 32. The nozzle 32 is acted upon a piezo electric device in a manner well known in the art (see, for example, U.S. Patent No. 3,512,172). The drops pass adjacent a charging electrode and then through an electrical deflection field (not shown). Ink flows to the nozzle 32 by way of a flexible conduit 11 from a pressurized reservoir 20 which is usually located remotely from the print head 26. If desired, the high pressure reservoir 20 may supply ink to several such print heads 32. The high pressure reservoir 20 is supplied with ink by various suitable means, many forms of which are known in the art. Typically, a recirculation system will include an ink drop return conduit 34 to return unused ink drops to a return ink reservoir 18 using vacuum pressure. Typical ink recirculation systems also include means for replenishing ink and solvent in order to make up for depletion during operation. According to the present invention, an ink concentration reference chamber 10 is positioned between the fresh ink supply and an ink flow control value 28. Mounted on the bottom exterior of the base of the reference chamber 10 is an acoustic transducer 12, which emits an initial acoustic pulse through the fresh ink to a reflector 14, which in this case may be the top of the chamber. A reflection occurs at the reflector 14 and the return pulse is detected by the transducer 12. The time delay required for the reflection and return of the return pulse is a function of the velocity of sound through the fresh ink. The resultant information is used as one input to the control system in Figure 2. It is also within the teachings of the present invention to measure other acoustic properties of ink utilizing acoustic sensors that relate ink solids concentration to ink density and ink viscosity for example. A transducer 15 or 13 is also mounted on the bottom of the high pressure ink reservoir 20 or the return ink reservoir 18 (or both) respectively. For these reservoirs acoustic reflection to generate a return pulse can be provided by a solid surface or a change in the acoustical impedance of the fluid column produced, for example, by a change in diameter at a point 16 in reservoir 18 and at a point 21 in reservoir 20. Preferably, the ink concentration reference chamber 10 and the reservoirs 18 and 20 are constructed so that the acoustical paths through the ink are identical in length, thereby obviating the need to compensate for chamber geometry, etc. The concentration reference signal from transducer 12 and the return reservoir concentration signal, reflected from the constriction point 16 from the return reservoir transducer 13, are fed to the closed loop control system depicted in Figure 2. Any difference in the two signals will be due to evaporation of solvent from the ink in use and generates an error signal for the controller 40, which in turn generates a solvent-add signal for operating the solvent flow control valve 30. Solvent is thereby added as needed to the ink return system to maintain the reservoir 20 ink supply substantially identical in concentration to that present in reference chamber 10. The controller 40 may be a solid state logic system or a programmed computer as, for example, a microprocessor computer system of the type typically used for process control. As the ink in the return reservoir is diluted with solvent, its sound velocity begins to match the sound velocity in the control chamber. This reduces the magnitude of the error signal. In turn, this reduces the rate of addition and ultimately terminates the flow of solvent. There is an additional benefit from using the sound transducers according to the invention. Float-based sensors such as used in the prior art are vulnerable to errors caused by mechanical binding, triggering errors, hysteresis and ink foam 24 as shown in Figure 1. Solid state measurement of fluid level external of the reservoir avoids these errors. Without any additional hardware, fluid levels can be measured and regulated. In either reservoir 18 or 20 the transducer 13 or 15 respectively will receive a second return pulse reflected from the liquid surface, for example 22 in the return reservoir 18. A fluid level controller using the second return pulse data from the transucer 13 can maintain optimum levels in the reservoirs by controlling the supply of fresh ink through flow control value 28. Figure 3 shows the transmitted and received pulses as described herein. It will be apparent to those skilled in the art that one controller 40 can perform both functions, that is, regulate the addition of solvent and fresh ink to the system. The timing of the second return pulse relative to the initial pulse defines a time interval which correlates with the ink level in the return reservoir 18 for transducer 13 or the high pressure reservoir 20 for the transducer 15. The time interval may be compared with a reference value stored in a controller memory and the result of that comparison used to control operation of the fresh ink valve 28 in the same manner as the solvent value 30 is operated. While preferred embodiments of the present invention have been illustrated and described, it will be understood by those of ordinary skill in the art that changes and modifications can be made without departing from the invention in its broader aspects, and in particular although the embodiment described senses speed of sound changes using a single transducer by a time of transit reflection technique, it would be possible to sense speed of sound changes using two transducers by a direct path time of transit technique.

Claims: CLAIMS 1. Ink control apparatus for a drop marking system which includes a reservoir, a printhead receiving ink from the reservoir, means for returning unused ink to the reservoir, the apparatus comprising: a) a solvent supply including a valve for controlling the addition of solvent to the ink in the reservoir; b) a reference chamber contain fresh ink and characterised by further comprising; c) an acoustic transmitter associated with said reference chamber oriented to transmit acoustic pulses through the ink and an acoustic receiver associated with said reference chamber arranged to detect said acoustic pulses after travelling through the ink, the time delay between transmission of a pulse and its reception being a function of the solids concentration of the ink in the chamber; d) a second acoustic transmitter associated with said reservoir oriented to send acoustic pulses through the ink and a second acoustic receiver associated with said reservoir arranged to detect said acoustic pulses after travelling through the ink, the time delay between transmission of a pulse and its reception being a function of the solids concentration of the ink in the reservoir; e) means for comparing the time delays of echo returns from the reference chamber and the reservoir and for controlling operation of the solvent supply valve as a function of the said difference; whereby solvent is added to the ink received in the reservoir as necessary to keep the solids concentration therein substantially the same as the ink in the reference chamber. 2. Apparatus as claimed in claim 1 in which the drop marking system includes a pressure reservoir and a return reservoir, the printhead receiving ink from the pressure reservoir and the unused ink being returned to the return reservoir and means for communicating ink from the return reservoir to the pressure reservoir and in which solvent is added to the ink received in the return reservoir to keep the solids concentration therein substantially the same as the ink in the reference chamber. 3. Apparatus as claimed in claim 2 in which the second acoustic transmitter and receiver are associated with the return reservoir. 4. Apparatus as claimed in claim 2 in which the second acoustic transmitter and receiver are associated with the pressure reservoir. 5. Apparatus as claimed in any preceding claim in which the first acoustic transmitter and first acoustic receiver are combined in a single first acoustic transducer. 6. Apparatus as claimed in any preceding claim in which the second acoustic transmitter and second acoustic receiver are combined in a single second acoustic transducer. 7. Apparatus as claimed in Claim 5 in which said reference chamber incorporates a reflector surface positioned opposite the transducer to produce said echo return after the pulse passes through the ink. 8. Apparatus as claimed in Claim 6 wherein said return reservoir includes a first portion connected to a second portion of dissimilar physical configuration, said echo return being produced by the discontinuity at the point of connection of the two portions. 9. Apparatus as claimed in Claim 6 wherein said pressure reservoir includes a first portion connected to a second portion of dissimilar physical configuration, said echo return being produced by the discontinuity at the point of connection of the two portions. 10. Apparatus as claimed in claim 8 or claim 9 in which the or each first portion is of substantially equal height to said reference chamber, whereby the echo returns from each can be directly compared, without need for calibration for any difference in volume or path length through the ink. 11. Apparatus as claimed in Claim 6 and further including means for controlling the addition of fresh ink to the return reservoir, said means comprising: a) a fresh ink supply and an ink valve associated therewith for communicating ink to the return reservoir; b) means for comparing the time interval between transmission of an acoustic pulse and receiving an echo return in the return reservoir related to the ink level in said return reservoir with a value representative of a desired ink level and for operating said ink valve to add fresh ink as necessary. 12. Apparatus as claimed in Claim 6 and further including means for controlling the addition of fresh ink to the return reservoir, said means comprising: a) a fresh ink supply and an ink valve associated therewith for communicating ink to the return reservoir; b) means for comparing the time interval between transmission of an acoustic pulse and receiving an echo return in the pressure reservoir related to the ink level in said pressure reservoir with a value representative of a desired ink level and for operating said ink valve to add fresh ink as necessary. 13. Ink control apparatus for a drop marking system including a reservoir to supply ink to a print head that has at least one orifice for projecting a stream of droplets towards the surface to be marked, the individual droplets being electrically controlled and directed towards the surface to mark it or to a catcher to be captured and returned to the ink reservoir, and having a source of fresh ink and a source of solvent which is selectively connected to the ink reservoir to replenish it and characterised by ink comprising; a) first means measuring acoustic properties of the fresh ink to obtain a reference signal value relating to ink solids concentration. b) second measuring means measuring the acoustic properties of the captured ink to obtain a signal value representative of the ink solids concentration in the captured ink. c) means for comparing said reference signal value and said representative signal value to generate an error signal proportion to the solids concentration variation between the captured ink and the fresh ink, and d) means for controlling the addition of the solvent in response to said error signal whereby the solids concentration of the ink and the ink reservoirs are maintained substantially constant. 14. A method for maintaining print quality utilizing an ink jet printer which has a reservoir to supply ink to a print heat that has at least ' one orifice for projecting a stream of droplets toward a surface to be marked, the individual droplets being electrically controlled to be directed toward the surface to mark it or to a catcher to be captured and returned to the ink reservoir, and having a source of fresh ink and a source of solvent which are selectively connected to the ink reservoir to replenish it, characterised by comprising the steps of; a) measuring acoustic properties of the fresh ink to obtain a reference signal value related to ink solids concentration, b) measuring the acoustic properties of the captured ink to obtain a signal value representative of the ink solids concentration in the captured ink, c) comparing said reference signal value and said representative signal value to generate an error signal proportional to the solids concentration variation between the captured ink and the fresh ink, and d) controlling the addition of the solvent in response to said error signal, whereby the solids concentration of the ink in the ink reservoir is maintained substantially constant.

Inventors: ANDERSON PHILIP DAVID, ANDERSON, PHILIP, DAVID
Applicants: GEN ELECTRIC CO PLC, THE GENERAL ELECTRIC COMPANY, PLC
Application year: 1994
Main classification: B41J2
IPC codes: B41J2, G01N29